RFR: JDK-8061259: ParNew promotion failed is serialized on a lock

Jungwoo Ha jwha at google.com
Thu Oct 30 17:24:09 UTC 2014


PTAL
http://cr.openjdk.java.net/~rasbold/8061259/webrev.02/


On Thu Oct 30 2014 at 12:28:19 AM Bengt Rutisson <bengt.rutisson at oracle.com>
wrote:

>
> Hi again,
>
> One more minor thing.
>
> The methods has_promotion_failed(), set_promotion_failed() and
> reset_promotion_failed() are protected but they could be made private
> instead.
>
> Thanks,
> Bengt
>
>
> On 2014-10-30 08:09, Bengt Rutisson wrote:
>
>
>
> Hi Jungwoo,
>
> On 2014-10-29 23:51, Jungwoo Ha wrote:
>
>  PTAL
> http://cr.openjdk.java.net/~rasbold/8061259/webrev.01/
>
>  I've looked a bit at the webrev. A couple of comments:
>>
>> Why do you use OrderAccess methods for writing and reading the
>> _has_promo_failed flag in has_promo_failed() and set_promot_failed() ?
>>
>
>   I think that has no effect on x86, but I assumed that processors with
> weak memory model may want ordering of set/reset/has call.
>
>
> You don't need the OrderAccess methods for the weak memory models here
> either. You just race on reading the variable and if you see the "wrong"
> value you eventually take a lock (which will order all memory accesses) to
> read the variable properly.
>
> By removing the use of OrderAccess you can make
> ConcurrentMarkSweepGeneration::_has_promotion_failed a bool instead of a
> juint which simplifies the code a bit.
>
>
>  Can we write out the full word "promotion" instead of just "promo" in
>> the variables and methods?
>>
>
>   Done.
>
>   Can we change the name of the flag from UseCMSFastPromotionFailure to
>> CMSFastPromotionFailure? Most CMS flags start with CMS and I don't think we
>> need the "Use" prefix.
>>
>
>   Done.
>
>  What do you think about making the flag true by default? At least for
>> JDK 9. If we decide to backport to JDK 8 or 7 it might be a good idea to
>> keep the default value as false.
>>
>
>   Done.
> Let me know if there is anything for me to do to backport to JDK8 and 7.
>
>
> I think this fix would be worth backporting to JDK 8. I don't think there
> is much action required on your side. I created a backport bug for JDK 8
> just to make sure that we don't forget it. It will be a little while before
> the 8 update repos are in a state to accept enhancements again. So, it is
> nice to have the backport bug to keep track of this.
>
> Backporting to JDK 7 requires some more work. Unless you have good
> arguments for why it is important to backport this to JDK 7 I don't think
> it is worth doing.
>
>
>  Did you find the information provided by _fast_promo_failure_hitcount
>> useful for debugging? If it not too useful I would consider removing it
>> since it is cluttering up the code a bit.
>>
>
>   I removed it.
> It was useful to for development, but I think it is no longer needed.
>
>
> Great. Thanks.
>
> One more comment. This code comment appears in two places just after we
> have taken the lock.
>
> 3365   if (CMSFastPromotionFailure && has_promotion_failed()) {
> 3366     // Caller must have checked already without synchronization.
> 3367     // Check again here while holding the lock.
> 3368     return NULL;
> 3369   }
>
> There is actually really no requirement that the caller must have checked
> has_promotion_failed() before calling the method. That's just an
> optimization. I think the first comment can be skipped and we just leave
> the second comment "// Check again here while holding the lock.". I would
> also suggest moving that comment up to the line just before the if
> statement.
>
> Thanks,
> Bengt
>
>
>
>
>
>
>
>
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